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Kern County, Calif., producers simplify soil moisture monitoring

Not surprisingly, the soil is wet just below the surface. December rain moistened San Joaquin Valley soils a bit.

While that may be a bit reassuring, he knows making a judgment about how much water is available to those vines with a scuff from his boots is like judging a novel by its cover.

However, an arm’s length away from where he is standing tacked to a grape stake is an open book revealing the whole water story of that vineyard down to five feet for the past five weeks.

All Wilson had to do was push a button on an AM400 data logger from MK Hanson and the 1.5 by 3-inch LCD screen reveals to him what the soil moisture "tension" is in the soil beneath those vines at an18-inch depth over the previous five weeks. Pushing the button a second and third time will give him the same information for soil depths of three and five feet.

Information on the data logger is coming from Watermark electrical resistance blocks, three-inch long cylinders that use electrodes to estimate soil moisture tension at three different levels. Attached to the bottom of half-inch PVC pipe, they operate on a principle similar to a tensiometer or gypsum block.

Simple, inexpensive

"What makes the Watermarks and data loggers different is that the system is inexpensive and easy to use. It costs about $600 for six probes and the data logger," said Blake Sanden, University of California Cooperative Extension irrigation and agronomy farm advisor for Kern County. "The data logger is the only one of its kind that does not require downloading to a computer to see recent changes in soil moisture. You get a five-week history immediately displayed on the screen. Of course, you can download the data into a laptop."

It has proven to be one of the simplest and least expensive soil moisture monitoring systems yet.

It is just one of many new technology tools growers and consultants are utilizing to measure soil and plant water levels.

Wilson has used them all from neutron probes to pressure bombs for measuring plant stress to muscle-powered soil augers.

"Other than a few technical managers working for some big ag companies, I don’t know of a grower/farm owner who has personally invested as much time and energy looking at irrigation management," said Sanden, who first began working with Wilson on irrigation monitoring and management almost a decade ago.

Water is an increasingly costly agricultural input in California and Arizona, making water efficiency go directly to the bottom line.

However, the importance of water management reaches much further than that. Groundwater protection is looming as a bigger issue each day and that goes directly to fertilizer management, making sure water and nutrients stay within the useable root zone for utilization by plants.

Crop quality factor

While maximizing yields is directly related to good water management, crop quality is becoming a bigger factor in this era when quality may sell a crop when poor quality nets rejection or deep price discounts.

Wilson has been a wine grape grower in Kern County for four decades. He’s has seen the bottom of three major grape price troughs in that period of time. The one the industry is now in is deeper than any of the others, he said.

Wilson expects vineyards to continue being dozed out in the coming year. What’s left will have difficulty finding winery homes over the next few seasons unless the quality is what wineries want.

Quality is determined by many factors, but deficit irrigation at the correct time in the summer growing cycle is one that is getting a lot of attention lately. One of the complaints from wineries is that SJV producers historically have gone for tonnage rather than quality and have irrigated toward that goal.

"In general, if we can give berries more light — not necessarily heat — we can improve color in red varieties. And, if color improves, flavor and quality can improve," he said.

That involves canopy management; holding down vegetative growth.

"Forty years ago Thompson seedless was a three-way grape in the San Joaquin valley; wine, table grapes and raisins and everyone farmed all grapes like they were Thompsons," he said.

That is no longer the case as growers have planted more varietal wine grapes. "People are becoming more quality conscious in producing wine grapes in the San Joaquin Valley. That involves picking the right varieties for the valley.

Fighting varietals

"A lot of the new, fighting varietal acreage was planted in recent years and some are doing well but others are not. Syrah and Ruby Cabernet are two varieties that are making good valley wine. Chardonnay out of the valley can make decent wine," Wilson said.

Southern and Central San Joaquin Valley wine grapes may be considered bottom rung in the hierarchy of California wine, but Wilson, like so many valley grape growers, points out that 60 percent of the state’s wine grape production comes from this area.

"Everyone talks down about the valley, but the wine from our grapes finds its way into wines sold out of other areas of the state," said Wilson, a director of the newly formed Central California Winegrowers Association. CCWA was organized to improve the valley’s wine image while also encouraging producers to enhance the quality of their grapes.

"Quality is achievable in the valley — if we pay attention to what we are doing," he said.

Deficit irrigating is one way to achieve that. But, timing has to be right. Wilson said that period from bloom to verasion is when he wants to manage canopy vigor by forcing the vine to be "thrifty" rather than go vegetative.

"When you start irrigating less than the full evapotranspiration rate, you are running some risks," said Wilson. "We have learned a lot about this from the Australians, but there is a story out of Australia about the risks involved in deficit irrigating."

The seasons are opposite Down Under. It’s hot there during December. One year a producer took a four-day Yuletide holiday and when he came back his pump had broken down and he lost half his grape crop in his vineyard where he had been deficit irrigating to improve quality.

Hands-on project

"If you want to improve quality with deficit irrigating, you have to be hands-on in irrigating," said Wilson.

Watermark sensors wired to in-field data loggers make it easier.

Wilson has installed one logger with three sensors in each of two locations in four 40-acre blocks.

"The idea is to have adequate moisture in the soil profile without flushing water through the bottom of the root zone," said Sanden.

Wilson does not rely on the soil moisture sensors alone. He still contracts for neutron probe readings and will use a pressure bomb to gauge water uptake of vines.

"The soil-based measurement may show adequate soil moisture, but for some reason the vine will not take it up. Measuring leaf water potential will help you identify those issues," said Wilson.

"We try to use as many tools as we can," he said.

Wilson irrigates with flood and surge irrigation as well as drip. He estimates that with new irrigation monitoring technology and new deficit irrigation scenarios, there is an "opportunity" to reduce water use by 25 percent from what it was just a decade ago.

"Drip has made a tremendous difference in how we manage water," Wilson said. "Most of our newer vineyards are on drip."

Same with less

"Water is certainly a cost, but from an ecological standpoint you never want to waste water. What we strive for is to do the same job with less water."

Reduced or better managed water use also affects other parts of the vineyard operation. "For example, if we find through soil monitoring that we have adequate soil moisture in the spring, we can do a few more operations like mildew control or weed management before we start irrigating."

Sanden has been working with about a dozen Kern County producers across several crops over the past two seasons using these sensors and the Hanson data loggers.

"Farmers know that knowledge is the key to more efficient irrigation practices," said Sanden. "Knowing for sure what is in the crop root zone at any given time tells you whether you are putting on too much or too little water to meet crop needs."

However, just as important today is crop quality related to water management.

"Proper irrigation management at the right time is the key to increasing fruit set and quality in many crops. Wine grapes are one and canning tomatoes is another," he said.

"There is evidence that you can improve fruit set in tomatoes by a little bit of deficit irrigating at the right time," he said.

"Even with the help of plant growth regulators like Pix, most of our new high yielding cotton varieties benefit from proper irrigation timing and some mild stress to set bold. You never want cotton to become over-lush," he said.

"Irrigation management is the key to keeping the growth manageable. However, once cotton is loaded and it hits those July temperatures, you want to make sure you have sufficient water to avoid stress. Again, that requires knowledge of water availability in the soil profile."

There is also some data to indicate deficit irrigating pistachios will result in weaker shells and perhaps more desirable shell splits.

"Blackeyes can tolerate a little bit of deficit irrigation until first flower," said Sanden.

Alfalfa, melons

Conversely, crops like alfalfa and melons do not tolerate deficit irrigation.

"With alfalfa you absolutely want to make sure it has all the water it can use, but at the same time you don’t want to waste water. Water-logging alfalfa is just as detrimental as deficit irrigating the crop," said Sanden.

Irrigation is often called more art than science. There is a mystery about it because short of backhoeing fields weekly to visibly look at soil moisture content, here is no way producers can tell for sure what is beneath the surface.

Growers must relay on some sort of monitoring technology.

While you can debate the costs, reliability and accuracy of available technology, one thing is for sure; the days of irrigating when your neighbor irrigates as gone the way of high commodity prices.

Sanden and Wilson believe the inexpensive Watermark sensors are reliable and the easiest to use for immediate in-field feedback.

"They are not foolproof. They require maintenance. They are less reliable in certain soil conditions — like real sandy soil. It’s not always easy to find the right place to put sensors on some crops. Furrow-irrigated melons on wide beds are an example of that," said Sanden.

However, Sanden said the ease of use and low cost represent a price breakthrough in taking some of the mystery out of irrigation management.

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